Fuel cells are well known and are commonly used to produce electrical energy by means of electrochemical reactions. Comparing to the conventional power generation apparatus, fuel cells have advantages of less pollutant, lower noise generated, increased energy density and higher energy conversion efficiency. Fuel cells can be used in portable electronic products, home-use or plant-use power generation systems, transportation, military equipment, the space industry, large-size power generation systems, etc.
According to the electrolytes used, fuel cells are typically classified into several types, e.g. an alkaline fuel cell (AFC), a phosphoric acid fuel cell (PAFC), a molten carbonate fuel cell (MCFC), a solid oxide fuel cell (SOFC) and a proton exchange membrane fuel cell (PEMFC). Depending on types of the fuel cells, the operation principles are somewhat different. For example, in the case of a direct methanol fuel cell (DMFC) which has the same structure as the PEMFC but uses liquid methanol instead of hydrogen as a fuel source, methanol is supplied to the anode, an oxidation reaction occurs in the presence of a catalyst, and protons, electrons and carbon dioxide are generated. The protons reach the cathode through the proton exchange membrane. Meanwhile, in the cathode, oxygen molecules take electrons from the anode and are reduced to oxygen ions by reduction. The oxygen ions react with hydrogen ions from the anode and thus produce water.
As know, an individual fuel cell unit supplies limited voltage (approximately 0.4 V). For a purpose of offering a sufficient operating voltage to an electronic product, a plurality of fuel cell units should be connected in series so as to form a fuel cell assembly. Depending on the arrangement of the fuel cell units, the fuel cell assemblies can be divided into two types, i.e. a stacked fuel cell assembly and a planar fuel cell assembly.
Referring to FIG. 1, an exploded view of a conventional stacked fuel cell assembly is illustrated. The stacked fuel cell assembly 10 comprises at least two membrane-electrolyte assemblies (MEAs) 11, a bipolar plate 12 located between two adjacent MEAs 11 and two electrode plates 13 and 14 at opposite ends of the cell assembly. Each MEA 11 includes an anode 111, a proton exchange membrane 112 and a cathode 113. The bipolar plate 12 comprises a plurality of channels 121 for flowing fuels and oxygen molecules therethrough. However, since the stacked fuel cell assembly 10 requires a large amount of cell units to be assembled in a stack form, the thickness and the weight thereof are considerably high. Therefore, the usage of such stacked fuel cell assembly is restricted in some situations.
Referring to FIG. 2, a planar fuel cell assembly 20 comprises a metal frame 21, a plurality of membrane-electrolyte assemblies (MEAs) 22 and two electrode plates 23 and 24 at opposite ends of the cell assembly. Likewise, each MEA 22 includes an anode, a proton exchange membrane and a cathode (not shown), and is embedded in the corresponding openings 211 of the frame 21. Furthermore, two current collectors 212 are disposed at one side of the frame 21 as the current output terminals of the planar fuel cell assembly 20. Each of the electrode plates 23 and 24 comprises channels 231 for flowing fuels and oxygen molecules therethrough. However, the metal frame 21 used in the planar fuel cell assembly 20 is both bulky and weighty. In addition, the procedure of aligning the MEAs 22 in the corresponding opening 211 of the frame 21 is complex and time-consuming. Such planar fuel cell assembly 20 is costly to manufacture, and also contribute a substantial weight and volume to the overall fuel cell assembly. In other words, such planar fuel cell assembly fails to be used in portable electronic products.